There are many industrial fields today that require powders. Some types of concrete require finely ground limestone, clay, and/or sand. The food industry requires many items to be ground including wheat, corn, rice, and the like. Some paint products also require ground pigments to provide color. Ceramics are made from ground materials. Metallic pigments require metals to be ground. There are many other industrial products and processes that require ground materials. As shown, many different industries require materials to be ground into a finer state. The different industrial and product requirements call for different degrees of fineness, depending upon the particular purpose of the powder.
One way in which materials are ground and/or mixed is through the use of ball mills. Ball mills typically include a chamber where the grinding and/or mixing takes place. Usually, the material to be ground or the materials to be mixed are placed in the chamber along with many steel or ceramic balls. The mill is then rotated, spun, or otherwise agitated to create a tumbling action within the chamber between the material and the balls. The balls hit one other and, as a result, grind and mix material that is found in between the balls. Also, the shearing action between the material and the balls further serves to grind the material. In addition, the shearing action between the material and the container walls effectively grinds the material into a finer material. After a time, the material within the ball mill is thus ground into a finer state and/or is better mixed.
As suggested earlier, the fineness of the powder varies according to the particular need. The finer the powder required, the smaller the particles need to be ground to produce the powder. The production of submicron-size powder is receiving greater attention because of the increasing demand for structural ceramics, magnetic materials, electronic packaging materials, and metal-ceramic composites. At the present time, such fine powders may be generated using, for example, ball mills, stirred ball mills, and vibration mills. However, these types of mills exhibit limitations when used to produce powders which are very fine. In this regard, such mills are typically only capable of producing a powder having a minimum particle size of about one micron (1 .mu.m) after several hours of grinding.
One reason for this minimum particle size limitation is the fact that particles are weakly confined in the breakage zone of these mills. The breakage zone is generally the interstices of the ball mass within the mill. Simply stated, the particles reside in the interstices between the balls, and are confined only by the weight of the ball mass. Accordingly, the particles may easily escape from this zone and avoid being ground to the degree they would be if they were within the breakage zone.